Project Summary Consistent with its functions in maintaining barrier integrity and preventing infection, the skin is home to a number of specialized T cell populations that not only combat infection but also work in concert to help maintain normal tissue homeostasis and promote wound repair. However, despite significant advances in understanding cutaneous immunity, the specific functions of different populations of cutaneous T cells, their roles in maintaining normal skin homeostasis, and contributions to inflammatory diseases of the skin remain poorly understood. In this respect, reliance on animal models can be problematic due to fundamental structural differences in the skin in humans vs. mice, and a lack of direct correspondence between cutaneous T cell populations in these species. Additionally, although skin-tropic T cells can be readily identified in peripheral blood based on their expression of the cutaneous lymphocyte antigen (CLA), the developmental and functional relationship of these cells in the blood with the different populations of T cells in the skin is still poorly understood. We have identified a novel population of CLA+CD4+ T cells in the peripheral blood of healthy subjects that expresses the CD103 integrin and produces the novel cytokine combination of IL-22, GM-CSF and IL-13 upon activation (CD103+CLAhi cells). Interestingly, this cytokine combination is also produced by phenotypically similar epidermal CD103 CLA CD69 TRM in the skin itself. Based on the ability of IL-22 to promote + hi + keratinocyte proliferation, migration and production of antimicrobial peptides, and GM-CSF and IL-13 to act on fibroblasts, and to mediate the differentiation of monocytes and macrophages into `alternatively activated' cells that promote tissue repair, we hypothesize CD103+CLAhi T cells in the blood represent a recirculating fraction of CD103+CLAhi TRM, and that these cells help coordinate the host-protective and wound healing responses following tissue damage in the skin. As such, these cells have the potential to be manipulated therapeutically to promote sterile wound healing and to optimize the development and function of engineered tissue grafts. In this proposal, we will use in vitro analyses and innovative humanized mouse models to test these hypotheses, assessing the developmental origins and trafficking behavior of CD103+CLAhi cells and determining their function during cutaneous tissue-repair responses.